Under certain conditions, some materials can be plastically deformed without rupture well beyond their normal limits, a property called superplasticity. This property is exhibited by certain metals and alloys within limited ranges of temperature and strain rate. For example, titanium alloys are superplastic in the temperature range 1450.degree.-1850.degree. F.
Superplastic forming (SPF) is a fabrication technique that relies on superplasticity. A typical SPF process involves placing a sheet of metal in a tool, heating the sheet to an elevated temperature at which it exhibits superplasticity, and then using a gas to apply pressure to one side of the sheet. The pressure stretches the sheet and causes it to assume the shape of the tool surface. The pressure is selected to strain the material at a strain rate that is within its superplasticity range at the elevated temperature.
One advantage of SPF is that very complex shapes can be readily formed. In addition, the SPF process is generally applicable to single and multi-sheet fabrication, and can be combined with joining processes such as diffusion bonding to produce complex sandwich structures at a relatively low cost. The simplicity of the SPF process leads to lighter and less expensive parts with fewer fasteners, and higher potential geometric complexity. Applications of SPF include the manufacturing of parts for aircraft and other aerospace structures.
The SPF process is often used to form sandwich structures which have a complex exterior surface geometry and an integral internal structural core. This core is used to stabilize the exterior surface geometry and to provide shear strength and stiffness to the structure.
One method to superplastic form sandwich structures is described in Hamilton et al., U.S. Pat. No. 3,927,817. In Hamilton, a sandwich structure is formed by joining three metallic worksheets together at selected areas. The worksheets are joined by first masking the areas of the worksheets where joining is not desired. Masking prevents the masked areas from joining during SPF. The worksheets are then placed adjacent to each other and heated to temperatures at which diffusion bonding is possible. Pressure is applied to the worksheets in order to cause the worksheets to diffusion bond together at the locations where no masking agent was used. The joined worksheets are then placed in a forming tool having a surface that defines the exterior surface of the formed structure. A pressure differential is then applied between the interior and the exterior of the worksheets. This pressure differential, combined with heating the worksheets to superplastic forming temperatures, results in the upper and lower worksheets superplasticly deforming to the shape of the tool surface. As the upper and lower worksheets expand, the middle worksheet forms a core structure. The locations at which the worksheets are joined together define the subsequent shape of the interior core structure.
An example of a sandwich structure formed with the method described in Hamilton is shown in FIG. 1. The sandwich structure consists of a coresheet 10 that is alternately joined to an upper sheet 14 and a lower sheet 16 along joints 12. The Hamilton method is commonly referred to as the "three sheet method." The "three sheet method" produces a sandwich structure with an internal truss core. The truss is formed from coresheet 10 and has webs 11 that extend diagonally between upper sheet 14 and lower sheet 16. These diagonal webs provide very good shear support and shear stiffness for the sandwich structure.
One problem with the "three sheet method" is that it results in surface grooving 18 or surface nonuniformity along joints 12 where coresheet 10 attaches to upper sheet 14 and lower sheet 16. Surface grooving 18 results at joints 12 due to the forces exerted on upper and lower sheets 14 and 16 by coresheet 10 during SPF. These forces prevent upper and lower sheets 14 and 16 from fully forming into the shape of the tool surface. Surface grooving 18 must be filled or covered when a smooth outer surface is required. Additionally, surface grooving 18 reduces the strength and stiffness of the finished structure.
Surface grooving 18 can be minimized by making upper sheet 14 and lower sheet 16 thicker (e.g. three times thicker) than coresheet 10. This stiffens the upper and lower sheets and reduces the effects of the forces applied by coresheet 10. However, thicker sheets result in a much heavier structure unless expensive chemical milling operations are used to remove extra material in the sheets.
In addition to the problems associated with surface grooving 18, the "three sheet method" has other disadvantages. The use of masking agents to prevent joining of the worksheets is time consuming and expensive. Additionally, the masking agents prevent the formed structure from being used in locations where contaminants cannot be tolerated. As an example, masking agents prevent the "three sheet method" from being used in "wet areas" of an aircraft structure. Wet areas are locations that are filled with aircraft fuel. Masking agents would contaminate any aircraft fuel located in these areas.
Another commonly used superplastic forming method is described in two patents to Hayase et al., U.S. Pat. Nos. 4,217,397 and 4,304,821. As shown in FIG. 2, a sandwich structure fabricated with the Hayase method uses two coresheets 20 and 22 which are welded to one another at selected areas 24. An upper cover 26 and lower cover 28 are placed over the coresheets. The resulting assembly is placed within a tool having a surface that defines the exterior surface of the formed sandwich structure. In the Hayase method, the assembly is heated to superplastic forming temperatures and a pressure differential is placed between the interior and exterior surface of covers 26 and 28 resulting in superplastic forming of the cover sheets. A pressure differential is then placed between the interior and exterior of coresheets 20 and 22 resulting in superplastic forming of the coresheets. Superplastic forming of the coresheets continue until coresheets 20 and 22 contact covers 26 and 28 and diffusion bond to them. This method, also known as the "four sheet method," eliminates surface grooving because cover sheets 26 and 28 are formed prior to forming the coresheets.
The four sheet method results in a number of new problems. It produces a core structure with webs 23 normal to cover sheets 26 and 28. In some respects, this type of core structure is not as structurally efficient in shear as a core structure with diagonal webs which form a truss. The four sheet method also results in excessive thinning at the 90.degree. corners 30, formed at the location where coresheets 20 and 22 contact cover sheets 26 and 28. This excessive thinning is a result of forming the small radius of curvature corners 30 preferred in the Hayase method. Furthermore, a longer processing cycle than the "three sheet method" must be used in order to allow time for coresheets 20 and 22 to form corners 30. The increased processing time increases costs and decreases part output.
Both the Hayase method and the Hamilton method form sandwich structures having the same internal reinforcement or number coresheets throughout the sandwich structure. A uniform internal reinforcement limits the structural engineer's ability to tailor superplastic formed sandwich structures to applications that have differing loads throughout the structure. Therefore, the structural engineer must design the entire internal reinforcement for the peak structural loading expected in the part. This results in an increased weight and a loss of structural efficiency.